Study On Rare Earth Enrichment Mechanism Of Zhijin Phosphate Rock In Guizhou Province

Rare earth elements have crucial strategic and economic significance and are widely used in military,medical,metallurgical,petrochemical,new materials,agriculture,and other fields.Phosphorite-type rare earth deposits as a potential source of rare earth have gained much academic attraction in recent years.Phosphorite first appeared in the Paleoproterozoic,and were widely deposited from the Ediacran to the early Cambrian Era,a critical transition in atmospheric oxygen,ocean environment,and animal evolution in geological history.The early Cambrian was characterized by a major peak of phosphogenesis and bioturbated substrate(Cambrian substrate evolution).Phosphorite giants during this period were distributed in Asia,North America,Oceania,and Western Europe.Early Cambrian phosphorite deposits in China are predominantly distributed in South China,such as Guizhou and Yunnan.Among them,Zhijin Phosphorite Deposit has attracted full attention due to its high contents in rare earth elements(REE).Many works have been attributed to the geological features,host minerals for REE,and REE contents variation for the deposit.However,the enrichment mechanism for REE is still controversial.Zhijin Phosphorite Deposit is a huge size of marine sedimentary phosphate deposit in the early Cambrian in the southwestern margin of the Yangtze platform in China.The deposit is hosted within Gezhongwu Formation,which is at the bottom of the Qiongzhusi Stage(middle and upper part of the Xiaotan Stage)with overlying Niutitang Formation of the early Cambrian and underlying Dengying Formation of the late Ediacaran.The mineralogy in the deposit is characterized by carbonate fluorapatite(CFA)and dolomite,trace quartz,mica,and pyrite.Phosphatized small shelly fossils dominated by hyolithids,olivooids,zhijinitids,conodontomorphs,yubelichitids,camenitids,and diversified sponge spicules were found in the phosphate rock.Ichnological fossils are prevailed by Treptichnus and Palaeophycos in the deposit.Tidal bedding and soft-sediment deformation structures are widely developed in the Zhijin Phosphorite Deposit.Based on the field and indoor sedimentary and petrological studies on the Zhijin Phosphorite Deposit,major and trace element compositions of bulk rocks and dolomite,CFA,and phosphatized small shelly fossil were analyzed by LA-ICP-MS.Raman spectroscopy is applied to characterize the crystallinity of the CFA and the SSF.The results show that the contents of rare earth element in the phosphate rock deposits during the Cambrian period(average?REE 571 ppm,n=53)are significantly higher than those in Ediacaran(the average?REE 124 ppm,n=68).The CFA?Ce values(average 0.38,n=53)is significantly smaller than those in the Ediacaran(average 0.76,n=68).It is consistent with global Ce anomalies in carbonate rocks and other geochemical evidence.It means that the more atmospheric and oceanic oxygenation in the Cambrian period than that in the Ediacaran.We proposed that enhanced oceanic oxygenation and bioturbation during the early Cambrian increased phosphorus burial and decreased organic matter burial,which introduced elevated REE into porewaters uptaken by the CFA.The REE concentrations in phosphorite deposits in Kunyang,Zhijin,and Bahuang,which are hosted by the same Early Cambrian strata,show an increasing trend with the palaeogeographical settings from the shallow to the deep water.It is consistent with the REE concentration variation from modern seawaters and implies that sedimentary control on the REE concentrations in the phosphorites from the Early Cambrian.Furthermore,diversified spongy spicules found in the phosphate and geochemical evidence support the upwelling of phosphorus-and REE-enriched deep basin waters caused to REE concentration variation from the phosphorites in different settings.Soft-sedimentary deformation structures developed in Zhijin phosphorite deposit suggest that a long-lasting diagenetic process compared with Kunyang deposit.This proposal also lends support from trace element and REE geochemistry from the Zhijin deposit and Kunyang deposit.Zr and Th,common detrital proxies in chemical sedimentary rocks,have higher contents in Zhijin Phosphorite than in Kunyang.And PAAS normalized(Sm/Yb)_Nratio,an index indicating diagenetic MREE enrichment relative to the heavy REE,record higher values(average 1.4)in Zhijin Phosphorite)than those(average1.1)in Kunyang.REE analytical results for dolomite,CFA,and SSF from Zhijin Phosphorite conducted by LA-ICP-MS reveals that PAAS normalized patterns in dolomites are similar to the modern seawater.However,the CFA and SSF share the same patterns with MREE rich and HREE depletion,which is dissimilar to the contemporary seawater with LREE depletion and HREE rich.We speculated that the dolomite is prevailingly precipitated from oxic water column,and the CFA was formed in the suboxic porewater with the REE sourced from organic matters decomposed.Lower REE contents in SSFs compared to the collophanes are interpreted by their better crystallinity than collophanes,as indicated by Laser Raman Spectrometry data.A two-stage conceptual model for the REE enrichment in early Cambrian marine phosphorite,as exemplified by Zhijin Phosphorite Deposit,has been constructed.During the sedimentary period of the phosphate rocks,more oxygenated marine water combined with bioturbation led to the higher REE contents in the early Cambrian primary CFA(or its precursor)than in the Ediacaran.The suboxic environment of Zhijin and stratified ocean context in the early Cambrian of South China result in the higher REE contents in Zhijin Phosphorite than in Kunyang Phosphorite.Subsequent more lengthy diagenesis caused more REE enrichment in Zhijin Phosphorite than in Kunyang.